Fix incorrect rvec and tvec when using useExtrinsicGuess

[Jcparkyn]

Fixes #1556.

I also updated the test to use inputs that reproduce the issue (for many inputs, SolvePnP will still converge when the initial rvec and tvec are wrong). With the updated test and no change to calib3d.h, the rvec and tvec outputs are clearly wrong. With the fix (or without useExtrinsicGuess) they are correct.

SolvePnPRansac has a similar issue, but I didn't fix it here because:

• The API uses out parameters for rvec and tvec, so modifying them would be a breaking change.
• Because these are out parameters, it's obvious that useExtrinsicGuess won't work, so it's much less likely to confuse users than the normal solvePnP version.

[shimat]

Sorry for the late reply. Thank you!